Ethanol production through simultaneous saccharification and fermentation of switchgrass using Saccharomyces cerevisiae D5A and thermotolerant Kluyveromyces marxianus IMB strains

Hydrothermolysis pretreated switchgrass at 200 °C for 10 min was used in a simultaneous saccharification and fermentation (SSF) process using five thermotolerant yeast strains Kluyveromyces marxianus IMB 1, IMB 2, IMB 3, IMB 4, and IMB 5 at 45 °C and Saccharomyces cerevisiae D5A at 37 °C. SSF was carried out for 7 d using 5, 10, and 15 FPU/g glucan to determine the effect of decreasing cellulase loading on ethanol yield. The effect of initial pH on SSF by S. cerevisiae D5A was also investigated. Fermentation by K. marxianus IMB 1, IMB 2, IMB 4, and IMB 5 ceased by 72 h and fermentation by K. marxianus IMB 3 ceased by 96 h, while fermentation S. cerevisiae D5A continued for 7 d. At 96 and 120 h, IMB 3 and S. cerevisiae D5A had similar ethanol yields while the other K. marxianus strains were lower at a 95% confidence level. Final ethanol yields for IMB 3, IMB 1, IMB 5 strains were similar to one another, however, ethanol yield for S. cerevisiae D5A (92% maximum theoretical) was greater than all of the IMB strains except IMB 3 at a 95% confidence level. Reducing enzyme loading reduced ethanol yields for both K. marxianus IMB 3 and S. cerevisiae D5A. Reducing buffer pH from 5.5 to 4.8 reduced ethanol yields for S. cerevisiae D5A. This study shows that K. marxianus IMB 3 has potential for commercial use for ethanol production from cellulose in SSF processes with further improvement of its thermotolerance.